ohmmeter
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ohmmeter
Ohmmeter
A portable instrument for measuring relatively low values of electrical resistance. The range of resistance measured is typically from 0.1 microhm to 1999 ohms (&OHgr;). The ohmmeter solves quickly and easily a variety of measurement problems, including measuring the resistance of cladding and tracks on printed circuit boards, electrical connectors, and switch and relay contacts, as well as determining the quality of ground-conductor continuity and bonding, cables, bus-bar joints, and welded connector tags. See Resistance measurement
Ohmmeter
a direct-reading instrument for measuring electric true (ohmic) resistances. Among the types of ohmmeter are megohmmeters, teraohmmeters, and microhmmeters, which differ in the ranges of the resistances being measured. Ohmmeters are made with a permanent-magnet measuring head or a permanent-magnet quotient meter.
The operation of a permanent-magnet ohmmeter is based on measurement of the current flowing through the resistance being measured, when the voltage from the source is constant. To measure resistances in the range from several hundred ohms (Ω) to several megohms (MΩ), the meter and the resistance rx being measured are connected in series. In this case, the current I flowing through the meter and the deflection α of the moving part of the instrument are proportional: I = Cα = U/ro + rx); α = U/C(r0 + rx), where U is the voltage supplied by the source and r0 is the resistance of the meter. For small values of rx (up to several ohms), the meter and rx are connected in parallel. If U and C are constant, the deflection α depends on rx; therefore, the scale of the meter can be calibrated in ohms for easier readout. The error of such an ohmmeter is 5–10 percent of the length of the scale used.
An ohmmeter is frequently part of a multimeter, such as the VOM (volt-ohm-milliammeter). The bridge method of measurement is used in ohmmeters for work requiring great precision. Electronic amplifiers are used to increase the sensitivity of the meter and to improve measurement accuracy.
Electronic ohmmeters with digital readout of the resistance being measured were introduced in the 1960’s. Some ohmmeters can be connected to computers. The range of measurement of such ohmmeters is 1 mΩ to 100 MΩ or more; the error is 0.01–0.05 percent.
REFERENCES
Shkurin, G. P. Spravochnik po elektro- i elektronnoizmeritel’nym priboram. Moscow, 1972.Spravochnik po elektroizmeritel’nym priboram. Edited by K. K. Iliunin. Leningrad, 1973.
E. G. BILYK
ohmmeter
[′ō‚mēd·ər]Ohmmeter
A portable instrument for measuring relatively low values of electrical resistance. The range of resistance measured is typically from 0.1 microhm to 1999 ohms (&OHgr;).
The ohmmeter solves quickly and easily a variety of measurement problems, including measuring the resistance of cladding and tracks on printed circuit boards, electrical connectors, and switch and relay contacts, as well as determining the quality of ground-conductor continuity and bonding, cables, bus-bar joints, and welded connector tags. See Resistance measurement